The family of wireless, or mobile, telephones include cellular telephones, wireless office phones, cordless telephones, and Internet Protocol (IP) phones.
The traditional wireless telephone used in many homes is a cordless telephone. Cordless telephones transmit and receive radio signals from a base station that is connected directly to a publicly switched telephone network (PSTN) line. Cordless telephones can send and receive radio signals with analog or digital frequency modulation protocols. Modem cordless telephones have the capabilities of switching frequencies automatically to avoid interference with other cordless phones and to avoid interference caused by electronic devices such as computers. Cordless telephone systems have a range of radio signal transmission typically in the hundreds of feet from a base station.
Cellular networks were initially installed with analog transmissions using AMPS protocols. Modern cellular networks have developed into digital formats using protocols such as global services for mobile communications (GSM) and code division multiple access (CDMA). FIG. 1 illustrates a schematic diagram of a cellular telephone network 10. A base transceiver station (BTS) 12 may broadcast radio frequency (RF) signals 14 towards a general area or in all surrounding areas of the BTS, which is represented by area 16 in the schematic. Cellular telephone network systems utilize wireless communication channels 18, or paths, between a mobile telephone 20 and a radio antennae on a base station tower 12. Cellular transmission antennas are commonly placed on a base station tower 12 or high-rise structures to support a regional or nationwide cellular network. Packetized voice data (voice packets) 18 are transferred to the PSTN. A BTS 12 may have a maximum RF signal 15 transmission distance ranging from approximately five to ten miles or more from a transmission 12, which varies depending upon numerous factors including interference with structures, topography, or foliage, transmitting power, and signal attenuation. By placing multiple transmitters in the cellular network, telephone calls may be seamlessly passed from one transmitting tower to another through a “handoff” procedure.
A land-based type of local telephone system used by businesses, hospitals, and schools is called a private branch exchange (PBX). A PBX is private telephone system switch that serves a digital or analog telephone network within a specific location, such as an office building or campus. A PBX can route telephone calls within the PBX network. PBX switches also route calls externally to the PSTN. Each on-site telephone is wired to the PBX switch instead of an external PSTN line. The advantage of a PBX system is that numerous telephones can share far fewer PSTN trunk lines to place and receive external calls.
A wireless PBX network uses unlicenced frequencies and has the capability of internal dialing from one wireless telephone directly to other wireless telephones on the network as well as dialing out to the PSTN. Wireless PBX telephone handsets must be custom manufactured to incorporate unique features of a particular PBX system and are therefore not widely used. Wireless PBX phones require base stations with broadcasting and receiving antennas placed throughout an office building and in outdoor areas between buildings. The base stations are wired to specialized hardware and software within a PBX system.
Similar to roaming features on a cellular network, calls on a wireless PBX system are transferred between base stations/access points (APs)when the mobile handset is moved out of range of a current base station/AP and towards a base station/AP with a stronger signal. Handsets for wireless PBX systems have an upper range transmission range of approximately 150 feet, depending upon interferences.
An additional wireless phone system is the Digital Enhanced Cordless Telecommunications (DECT) phone. DECT is a standard for international digital radio access for cordless telephones. It is based on TDMA (time division multiple access) protocols and offers heightened security and low interference. The signal range for a DECT phone is approximately 50 to 300 meters, depending upon interferences. A DECT phone can operate using a base station in a home or office or from a network of several base stations. For example, a DECT phone can act as a PBX extension within an office. DECT handsets can communicate with other DECT phones through the base station. DECT technology has been combined with GSM to create a dual mobile handset where calls to a phone can be routed through either the DECT base station or through a mobile GSM network.
Telephone handsets can be integrated into local area networks that have network entities like proxy servers, media gateway controllers (MGC), media access gateways, or gatekeepers to route calls from the handset through a gateway using packet telephony under voice over Internet Protocols (VoIP). A media access gateway works in coordination with an MGC to connect the physical links of the LAN to external networks, including the PSTN, and for controlling media stream conversions between protocols of different systems as required.
A wireless local area network (WLAN) links wireless network devices up to a few hundred yards in distance, which makes WLANS useful for mobile network access in office buildings, warehouses, hospitals, and campuses. WLANs utilize RF signals or light signals to connect mobile devices to each other or to a centralized controller and transmit data between the physical devices. In 1997 the IEEE published standards for WLANs under the title of 802.11. The 802.11 standards contain different protocols that may use unlicenced 2.4 GHz and 5 GHz radio bands to transmit packet data. IEEE 802.11 enables mobile stations (e.g., endpoints) to communicate through a wireless network interface card directly with each other or with other stations through an access point. An access point is a centralized gateway providing message and power management and access to an external LAN and/or the Internet. IEEE 802.11 access products are sold with personal computers (e.g., wireless NICs), computer peripherals, print servers, and mobile devices such as laptops and personal digital assistants (PDAs).
There are a plurality of 802.11 standards that each use different frequency bands and have varying data transmission speeds. The original IEEE 802.11 standard supported wireless interfaces operating at speeds of up to 2 megabyte per second (Mbps) in the 2.4 GHz radio band. By using different modulation techniques, IEEE 802.11b raised the data transmission rates to 11 Mbps, while 802.11a supports up to 54 Mbps transmission rates at a 5 GHz frequency. The IEEE 802.11g is developing standards for data transmission rates of 54 Mbps at the 2.4 GHz frequency.
FIG. 2 illustrates a schematic diagram of an 802.11 enterprise network 22 that may be used to implement a wireless telephone system. An IP PBX switch 24 serves as a centralized gateway for processing voice calls placed through the network. Two wireless access points 26 (and 26′) are connected to an internal corporate Intranet 40, which is connected to IP PBX 24. The Internet 38 may be accessed through APs 26 and 26′, through intranet 40, and through IP PBX 24. Each AP 26 has a range of RF broadcast signals 28 represented by area 30. A wireless mobile telephone 32 can transmit and receive digital voice signals 34 through one or more APs 26 to the IP PBX 24. A wireless telephone 32 can place calls through the IP PBX switch 24 to the PSTN 40 or the call may be routed through the Internet 38, or other type of LAN, to a remote receiver. Multiple APs 26, 26′ provide an enterprise wide footprint that can be accessed by a wireless handset 32, depending upon natural signal attenuation of a broadcast signal and intereferences. An enterprise network typically has multiple APs distributed throughout an office or between multiple buildings so that the handset may be operated nearly anywhere in the network broadcast area 30. Therefore, a wireless handset can operate/function within an office with multiple APs or between APs in multiple office buildings with seamless handoffs between the different APs in the same network. Since the 802.11 network is traditionally a data network, a wireless endpoint, such as laptop computer 36, may access the Internet 38 through an AP 26′. In such a configuration, an enterprise 802.11 data network 22 is leveraged for VoIP calls and wireless data access by a wireless endpoint such as mobile phones or wireless network interface card in a computer 36.
The diagram in FIG. 3 illustrates the general assumption that an 802.11 wireless home network rarely has more than one AP 44 per household. AP 44 connects to router 53, which connects to either Internet 38 or connects to PSTN 40 through gateway 52. In a home network 42, RF broadcast signal area 46 is typically adequate from a single AP 44 unless a larger area of coverage is desired. In such a case, extender technology may be used to expand coverage, but using an expander would be the exceptional case. A wireless handset 48 sends voice signals 50 to an AP 44 that is networked into VOIP gateway 52. The VOIP gateway 52 sends voice signals to either the Internet 38 or to the PSTN 40. Laptop computer 54 may also connect to the Internet 38 through AP 44.
Many retail locations, airports, and coffee shops throughout the United States now provide “hotspot” wireless networks. FIG. 4 illustrates a hotspot network 56 with RF signal coverage area 58. Hotspots are typically 802.11 WLAN carriers that may be accessed through a wireless endpoint 60 that connects to an AP 62. The user is authenticated through an authentication server 63, such as a RADIUS server, in order to access Internet 38 through a subscription with an Internet service provider.
Conventional solutions to wireless voice networks include a single telephone that can theoretically roam between both an 802.11 WLAN network and a cellular network the two networks to place and receive calls, and a handset can detect when the user is within range of a signal from either network, with one network preferred over another depending on user preferences. Providing seamless roaming between a wireless LAN using VOIP protocols and a cellular network (e.g., to move from a wireless LAN network signal using IP protocols to a cellular network signal), however, creates complicated technical issues. Complex internetworking challenges between cellular and wireless LAN protocols have to be resolved. Further, cellular telephone network standards may have to be modified as well. The result is that the handset disconnects from a current network and must re-route a call to a completely different network with different protocols and different call setup procedures. Cellular network service providers will most likely be reluctant to embrace a system that disconnects from their cellular networks to place and receive calls, resulting in loss of revenue. The decision tree to determine when such a handoff or roam should occur, the need for interoperability between different cellular carriers and LAN-based networks, combined with irritation to a user who wishes to maintain a call in progress, are all problems with such a system.